Recently, the demand for portable electronic products such as notebooks, video cameras, cellular phones or the like has rapidly increased, and electric vehicles, energy storage batteries, robots, satellites have been actively developed. For this reason, high-performance secondary batteries allowing repeated charging and discharging are being actively studied.
Currently, nickel-cadmium batteries, nickel-metal hydride batteries, nickel-zinc batteries, lithium secondary batteries, and the like are used as commercial secondary batteries. Among them, lithium secondary batteries have little to no memory effect in comparison with nickel-based secondary batteries, and thus lithium secondary batteries are gaining a lot of attention for their advantages of free charging or discharging, low self-discharging, and high energy density.
A lithium secondary battery generally uses lithium oxide and carbonaceous material as a positive electrode active material and negative electrode active material, respectively. The lithium secondary battery includes an electrode assembly in which a positive electrode plate and a negative electrode plate respectively coated with the positive electrode active material and the negative electrode active material are disposed with a separator being interposed between them, and an exterior, namely a case, which seals and accommodates the electrode assembly together with an electrolyte.
Generally, a lithium secondary battery may be classified into a can-type secondary battery where the electrode assembly is included in a metal can and a pouch-type battery where the electrode assembly is included in a pouch of an aluminum laminate sheet, depending on the shape of the case.
Recently, secondary batteries are widely used not only for small-sized devices such as cellular phones but also middle-sized or large-sized devices such as vehicles and power storages. In particular, along with the exhaustion of carbon energy and the increased interest on environments, hybrid electric vehicles and electric vehicles attract attention globally, for example in US, Europe, Japan and Korea. In such a hybrid electric vehicle or electric vehicle, a battery pack for giving a driving force to a vehicle motor is the most essential part. Since a hybrid electric vehicle or electric vehicle may obtain a driving force by means of charging and discharging of the battery pack, the hybrid electric vehicle or electric vehicle ensures excellent fuel efficiency and exhausts no or reduced pollutants, and for this reason, hybrid electric vehicles and electric vehicles are used more and more. In addition, the battery pack of the hybrid electric vehicle or electric vehicle includes a plurality of secondary batteries, and the plurality of secondary batteries are connected to each other in series or in parallel to improve capacity and output.
Such parallel or series connection among secondary batteries may be determined in various ways depending on output, capacity, structure or the like of the battery pack in consideration of a device to which the battery pack is applied. Therefore, secondary batteries are connected in series or in parallel in various ways to configure a battery module, and the battery pack may include at least one battery module. In particular, the battery module may be configured so that three or more secondary batteries are connected in parallel, on occasions.
In order to connect three or more secondary batteries in parallel as above, electrode leads of the same polarity should be coupled to each other through a bus bar, and the electrode leads are frequently welded to stably maintain their coupled state.
In order to weld the electrode lead and the bus bar, laser welding is representatively used. Generally, in the laser welding, if a plurality of electrode leads, for example three or more electrode leads, are welded to a bus bar in an overlapped state, the welding reliability may not be ensured. In other words, as the number of electrode leads welded to the bus bar increases, the electrode leads are not welded agreeably to the bus bar, which may result in failure in welding. Also, even though welding is performed, the welded portion may be separated while the battery module is manufactured or used later. In particular, if the electrode lead is not properly welded to the bus bar and its welded portion is separated while the battery module is in use, the battery module may suffer from deteriorated performance or malfunction, and also the separated electrode lead may cause an electric short in the battery module, which may lead to fire or explosion.
Therefore, in an existing technique, when a plurality of secondary batteries, for example three or more secondary batteries, are connected in parallel, ultrasonic welding is frequently used, or a plurality of bus bars are used so that the electrode leads and the bus bars are laser-welded.
However, if such an existing technique is used, since the welding process is limited to the ultrasonic welding, the degree of freedom in designs and processes may be seriously reduced. In addition, the ultrasonic welding may not ensure a sufficient welding strength in comparison to laser welding and may demand more costs due to exchange of consumables such as a horn and an anvil. Moreover, if a plurality of secondary batteries are connected in parallel using a plurality of bus bars, the welding process becomes complicated, and the bus bars occupy a greater space in the battery module, which may increase costs.